Wireless communication systems, such as the third-generation (3G) of mobile telephone standards and technology are well known. Such 3G standards and technology have been developed by the Third Generation Partnership Project (3GPP). The 3rd generation of wireless communications has generally been developed to support macro-cell mobile phone communications. Such macro cells utilise high power base stations (NodeBs) to communicate with wireless communication devices within a relatively large geographical coverage area. Typically, wireless communication devices, or User Equipment (UEs) as they are often referred to, communicate with a Core Network (CN) of the 3G wireless communication system via a Radio Network Subsystem (RNS). A wireless communication system typically comprises a plurality of radio network subsystems, each radio network subsystem comprising one or more cells to which UEs may attach, and thereby connect to the network. Each macro-cellular RNS further comprises a controller, in a form of a Radio Network Controller (RNC), operably coupled to the one or more NodeBs. Communication systems and networks have developed towards a broadband and mobile system. The 3rd Generation Partnership Project has developed the so-called Long Term Evolution (LTE) and LTE advanced solutions, namely, an Evolved Universal Mobile Telecommunication System Territorial Radio Access Network, (E-UTRAN), for a mobile access network, and a System Architecture Evolution (SAE) solution, namely, an Evolved Packet Core (EPC), for a mobile core network. A macrocell in an LTE system is supported by a base station known as an eNodeB or eNB (evolved NodeB).
Current wireless communications networks operate using licensed radio spectrum in which multiple accesses to the communications resources of the licensed radio spectrum is strictly controlled. Each user of the network is essentially provided a “slice” of the spectrum using a variety of multiple access techniques such as, by way of example only but not limited to, frequency division multiplexing, time division multiplexing, code division multiplexing, and space division multiplexing or a combination of one or more of these techniques. Even with a combination of these techniques, with the popularity of mobile telecommunications, the capacity of current and future networks is still very limited, especially when using licensed radio spectrum.
The use of unlicensed radio spectrum may also be used by network operators in order to increase or supplement capacity. For example, a network based on the Long Term Evolution (LTE)/LTE advanced standards has an enhanced downlink that uses a Licensed-Assisted-Access (LAA) procedure to operate on unlicensed spectrum. All communication devices need to complete a LBT (Listen Before Talk) procedure before accessing an unlicensed channel and it has been agreed to classify LBT schemes into 4 categories. In particular, Category 4 specifies LBT with random back-off with variable size of contention window. A device monitors a channel to decide if it is clear or busy. To make this decision, a device needs to keep listening to the channel for a “backoff” period. During this “back-off period, a timer is decremented. If the channel becomes busy (e.g. energy is detected over a predefined threshold) then the timer is frozen until the channel is no longer busy whereupon the timer continues decrementing. When the back-off timer expires then the device can transmit. During the timer running period, a number of channel clearance assessments (CCA) are performed. This number is randomly drawn from a “contention window” (CW). The CW is initially assigned a minimum size CWmin which is then adjusted based on collision detection. CW size is increased by a predefined step when collision is detected (i.e. an unsuccessful transmission) until a maximum value CWmax is reached, or reset to a minimum value CWmin when collision is not detected (i.e. a successful transmission).
Currently, for LAA in LTE, DL (downlink) and UL (uplink) are implemented in different ways, and an eNB can start a DL transmission any time on any channel while UEs can only start an UL transmission on specific subframes of specific channels allocated by the eNB with UL Grant messages.
For downlink LBT, four different priorities have been defined each having different CWmin and CWmax values and different CW steps (amongst other parameters). A priority is selected according to the traffic type to be sent. For example, an instant message requires a short latency and so a priority with a small contention window can be selected so that the uplink transmission can start after a short LBT procedure.
Methods for updating downlink CW and uplink CW are known and certain techniques are described in 3GPP TS 36.213 V13. For example, in the downlink an eNB may send a number of transport blocks (TBs) to several UEs in a reference subframe. Subsequently all UEs indicate an Ack/Nack in the uplink control signalling (using a licensed channel) depending on whether the received TBs were decoded successfully or not. If 80% or more transport blocks are indicated as Nack, then the eNB increases its CW size by one step, otherwise the CW is reset to the permitted minimum value CWmin. In the uplink, a CW size may be managed by the eNB and indicated to a UE in the uplink grant message. For example, for Category 4 LBT and for PUSCH (Physical Uplink Shared Channel) transmission on an LAA SCell, a CW size may be adjusted per UE based on whether or not a reference subframe in an uplink transmission burst transmitted by the UE is successfully decoded at the eNB. In particular, if at least one transport block in the reference subframe is successfully decoded at the eNB, then the CW size is reset for all priority classes. Otherwise, it is increased to the next higher value for all the priority classes. Some methods rely on decisions being made in the UE.
Known techniques do have certain drawbacks however. For example, a false alarm can occur when a UE is not transmitting in the reference subframe due to LBT failure but another device (for example, a Wi-Fi access point) is transmitting on the channel in question. In such a case the eNB may believe that it has received an incorrect uplink transmission from the UE and as a result, indicate to the UE to increase the CW size (unnecessarily). This action will, disadvantageously, reduce the uplink throughput. A missed detection can occur when the UE did transmit a reference subframe but the eNB failed to detect it due to collision with transmissions from another device. With a missed detection, the CW size will not be increased accordingly (although it should be). This problem of missed detection has the further disadvantage of permitting contiguous interference with Wi-Fi signals whenever collisions keep happening without being detected. Some known methods rely on decisions being made at the UE such that the eNB does not know the exact value of the current CW size. This can lead to inefficient scheduling at the eNB. Some known methods ensure alignment of the CW size between the eNB and the UEs. However, a mismatch may still occur whenever an uplink grant message is lost.
It would be advantageous to provide a means for updating an uplink contention window size which mitigated at least some of the disadvantages of known systems and methods.